Structural and Thermoelectric Properties of Al$_{1+x}$Fe$_{2-y}$M$_y$B$_2$ (M=Ag, Ni, Sb, Ga, and Ge) Intermetallic Borides

TL;DR

This study investigates how doping AlFe$_2$B$_2$ with various elements affects its structure and thermoelectric properties, revealing that Sb doping notably improves thermoelectric performance by increasing the Seebeck coefficient and reducing thermal conductivity.

Contribution

The paper provides new insights into the effects of Ag, Ni, Sb, Ga, and Ge doping on the structural stability and thermoelectric performance of AlFe$_2$B$_2$, highlighting Sb doping as particularly beneficial.

Findings

Sb doping increases Seebeck coefficient and power factor.

Doping decreases thermal conductivity, enhancing thermoelectric efficiency.

Structural stability is affected by dopants, with Sb showing the most favorable effects.

Abstract

AlFe$_2$B$_2$, a ternary transition metal boride doped with Ag, Ni, Sb, Ga, and Ge was investigated for the phase constituents, microstructure, and thermoelectric properties. The parent compound with 20\% excess Al (Al$_{1.2}$Fe$_2$B$_2$) prepared by vacuum arc melting contains orthorhombic AlFe$_2$B$_2$ and FeB upon doping form additional phases such as Ag$_5$Al, AlNi$_3$, Al$_3$Ni$_2$, AlSb, and AlB$_2$. The structural parameters of the AlFe$_2$B$_2$ phase in Ag, Ni, and Sb-doped samples are nearly the same as the un-doped one. The lattice parameters and cell volume increase noticeably in Ga and Ge-doped compounds. The Ga-doped sample shows only FeB as a secondary phase; however, the AlB$_2$ phase is observed in Ge-doped AlFe$_2$B$_2$. The microstructure investigated in FEG-SEM with EDS shows the AlFe$_2$B$_2$ matrix is chemically homogenous in all samples, except the Ga-doped one, with uniformly distributed single or multiple secondary phases. The DSC-TG results show that all the dopants decrease the AlFe$_2$B$_2$ phase peritectic decomposition reaction temperature, indicating the structure destabilizes. The thermoelectric properties measured show that AlFe$_2$B$_2$ is an n-type compound with electrical conductivity in the range of 0.35-0.46$\times$10$^6$ S/m. The addition of Ag, Ni, Ga, and Ge marginally alters the Seebeck and electrical conductivity. The noticeable improvement in Seebeck coefficient and with negligible change in electrical conductivity resulted in a highest power factor of 0.4 mW/mK$^2$ for Sb-doped sample. The thermal conductivity of AlFe$_2$B$_2$, which is in the range of 6.4-9.1 W/m-K between 300 to 773\,K, decreases with Sb doping to 5.2-8.5 W/m-K resulting in a maximum zT of 0.04 at 773K.